1. Field of the Invention
The present invention relates to proper adjustment of air flow created by rotation of the disk in disk storage systems. More specifically, the invention relates to adjusting air flow in a hard disk drive by provision of an after-assembly component, thereby reducing disk flutter.
2. Background Art
FIG. 1 is a block diagram showing the essential elements and operation of a hard disk drive (HDD) 10. The HDD 10 incorporates a disk 12 therein. (There are cases where a plurality of disks are integrally formed into a stacked structure like 12' and 12". In the following description, superscripts ' and " represent similar members.) When the disk 12 is being rotated on a spindle 14 by a motor 16, data read and write operations can be performed by a positioning operation with respect to the disk 12.
More specifically, an actuator is rotatably attached so that a head 18 (18' and 18" which include a magnetic read sensor and a magnetic write transducer) can be positioned over the rotating disk surface 13 (13' and 13"). The actuator generally includes a suspension 20 (20' and 20") and an actuator arm 22 (22' and 22"). The actuator is rotated on a rotational shaft as a unit by an actuator drive mechanism 24 so that it is positioned over a desired data track on the disk surface. The rotation of this actuator mechanism 24 is controlled by a control unit 26. The actuator drive mechanism 24 typically employs a voice coil motor (VCM). In addition, the operation of the motor 16 and the read and write operations of the magnetic head 18 are controlled by the control unit 26.
FIG. 2 is a plan view of the HDD 10 shown in FIG. 1. As seen in FIG. 2, a housing 30 is required to enclose all parts shown in FIG. 1. The spatial arrangement of the parts in the depth direction of the paper surface is shown in FIGS. 6(b) and (c). Covers are provided on the bottom (depth side of the paper surface of FIG. 2) and top (on this side of the paper surface or on the upper side of FIG. 6(b) or (c)) to form an enclosure with exterior walls of the housing 30. Therefore, the air flow which is generated by rotation of the disk 12 experiences hydrodynamic mutual action between these exterior walls. The details of this mutual action will be described later.
The high density and large capacity in the HDD has been supported by techniques for a reduction in the spacing between the head and the disk and by high precision head positioning. For this reason, in the design of the disk rotary mechanism, hydrodynamic considerations, such as (a) rotational precision of a disk, (b) air flow and dust removal in the HDD, (c) a reduction in internal generation of heat and a uniformity in temperature, and (d) a reduction in disk flutter, are very important and are being taken as a matter of course. A description will hereinafter be made of these problems (a) through (d).
First, in the background art, there is a contrivance to address the aforementioned (b). A slider 18 to which the magnetic read sensor and the magnetic write transducer are attached is aerodynamically supported (or floated) with respect to the disk surface 13 by very thin air bearing flow. Hard disks must be kept in a dust-free environment because if dust particles get into the space between the disk surface and the slider, they will often destroy the data stored on the disk. Therefore, in the HDD fabrication process, hard disks are kept in an extremely clean room and are hermetically sealed so that a foreign substance does not get into the HDD. On the other hand, it cannot be avoided that internal dust particles will appear in the HDD later on. This is due to degradation of elements during the rotational sliding motion of a disk by a motor, the rotational sliding motion of a rotary actuator by a VCM, and the sliding motion of other elements.
Hence, a filter is often used to perform a cleaning operation to keep the interior of the HDD clean. In order to effectively remove dust particles with a filter, the air flow in the HDD is adjusted. For this reason, various contrivances for changing air flow can be found in background art. A pressure baffle 40 shown in FIG. 2 is an example of the contrivances and takes in air flow developed by rotation of a disk and filters air with a filter 50. Even if the pressure baffle 40 and the filter 50 are present, the disk 12 is substantially enclosed by the housing 30.
A contrivance to address the aforementioned (c) is known in background art. Various return paths for air flow are provided to cool internal parts which generate heat. Flow rate is increased at a place where a large quantity of heat is generated, thereby making the interior temperatures of the HDD uniform.
Now, consider a countermeasure for the aforementioned (a). The problem with the rotational precision can be due to, for example, ill-balanced disk weight distribution, a disk eccentricity from a rotational center, and deformation of the disk itself by motion resulting from rotation. In order to prevent the deformation of the disk caused by rotation of the disk, increasing the thickness of the disk to enhance its bending rigidity can be an effective solution. However, this conflicts with the miniaturization and weight reduction of the HDD, and therefore may not be said to be an effective solution.
Next, consider the aforementioned (d). For rotating (or moving) bodies, problems of a natural oscillation frequency cannot be avoided. The "disk flutter" used herein means the vibration of a disk which occurs at the resonance frequency of the disk. The disk vibration influences the precise positioning of a head, so in the case where thin and high-density disks must be used, further consideration to resonance becomes important.
In addition, the problem becomes noticeable in the case where the number of revolutions of a disk is high. When the number of revolutions of a disk is extremely high, the air flow developed by rotation of the disk is faster. When this high-speed air flow contacts the exterior walls of the HDD and the complicated structure of internal components, the air flow will become turbulent. Air flow having turbulence is called "turbulent flow", and it contains temporally and spatially irregular fluctuation. The turbulent flow gives successive change to the rotary motion of a disk, that is, successive motion. This is "excitation" with respect to the rotary motion of a disk. Particularly, it becomes important to consider influence on the disk flutter at the resonance frequency.
Now, consider how air flow is developed. In the case where a disk is rotating at a circumferential speed of v with respect to the circumferential direction of the disk (counterclockwise direction in FIG. 2), the disk surface gives a shearing action to air near the surface.
In FIG. 2, if the angular velocity of a disk is taken to be .omega. and the radial position on the disk surface is taken to be r, a relation of V=r.omega. will be established. That is, when r=0 (rotational center of a disk), V=0 and therefore no shearing action occurs, and when r=R (outmost circumference of a disk), V=R.omega.. Therefore, the maximum shearing action occurs at the maximum speed. Thus, the circumferential speed is gradually increased, as the radius of the disk is increased from the disk center toward the outer circumference. The air nearest to the rotational shaft (e.g., the spindle 14) forms laminar flow, because there is less shearing action between the disk surface 13 and the air mass since the circumferential speed is slower in the inner side of the disk than in the outer side. On the other hand, as the radius of the disk is increased from the rotational shaft toward the outer circumference, air flow on the disk surface is disturbed and therefore becomes turbulent flow.
As the disk is rotated, air near the disk surface flows from the rotational central portion (inner track) toward the outer circumference (outer track) due to a difference in circumferential speed and centrifugal force. For this reason, air pressure is high at the outer circumference of the disk and is low (negative pressure) at the inner circumference. Air mass is present between the disk and the exterior wall (or cover exterior wall) of housing 30; and in the case of a plurality of disks, air mass is present between adjacent disks (e.g., disks 12 and 12' or disks 12' and 12" in FIG. 1). For the aforementioned reasons, the air mass is drawn radially out to the outer side of the disk and is finally drawn out to the outermost circumference R. Thus, in the outermost circumference (circumferential edge) of the disk and the vicinities, the air flow is in a hydrodynamically complicated state.
Of course, air mass is disturbed not only by an increase in the disk surface speed which destroys a laminar boundary layer, but it can also be disturbed by the actuator extending between disks, including the suspension 20 and the actuator arm 22.
It is believed that a turbulence in air flow is mainly caused by the mixing of air flow going out of the disk surface and air flow (return flow) coming into the disk surface. This turbulence is believed to be conspicuous near the outermost circumference (circumferential edge) R of the disk. Since the housing and the cover are present, the return flow of air from the housing and cover is a phenomenon which cannot be avoided. The hydrodynamically mutual action between the disk outermost circumference and the housing 30 (or the cover) cannot be avoided.
If a flow straightening plate is provided at the outer circumferential portion of the disk, the mixing between air flow going out of the disk and the return flow can be reduced. Therefore, a straightening plate can reduce a turbulence in air.
In high density and large capacity HDDs, high precision is required in positioning a head over circular data tracks concentrically formed in the radial direction of the disk. That is, it is important for high precision head positioning to make rotations of a disk as uniform as possible. It is undesirable to excite disk flutter by turbulent flow.
In addition, stacked disks operate as a pump which attempts to separate air mass from the disk circumference. This pump operation and frictional loss associated with it are undesirable because it consumes a very large quantity of energy. Furthermore, there is a need to accelerate standing air near the circumferential edge of the disk to the circumferential speed of the rotating disk, and this requires additional energy for rotating the disk.
The adverse effect of air mass near the circumferential edge of the disk is undesirable from the standpoint of saving energy. Extra drive force must be given to the VCM to rotate the disk, and therefore power dissipation becomes large. The energy which is consumed due to air turbulence (vortex motion) caused by turbulent flow is also wasteful. If a flow straightening plate is provided near the circumferential edge of the disk, air mass can be separated from the circumferential edge of a rotating disk and the adverse effect of air mass can be suppressed.
The turbulence in air flow is a primary cause of acoustic noise. This is called aerodynamic sound and believed to be caused by unsteady air flow, for example, shearing flow such as unsteady vortex motion.
Although a shroud has been provided in background art as described above, in connection with the aforementioned problems (b) and (c), no attempt has been made to positively solve the aforementioned problems (d) and (a).
Since the aforementioned problems (a) and (d) are a serious problem associated with the assembly of the HDD, they will hereinafter be described.
FIG. 3 is a plan view for explaining problems which arise in HDD assembly sequence. As shown in FIG. 3, a portion of the housing is set at a predetermined angle range 70 so that the exterior circumference of a disk is shrouded. From the standpoint of aerodynamics, it is desirable to shroud a wider angle range of the circumferential edge of the disk. However, referring to FIG. 3(a), if a shroud is provided in an angle range which is too wide, the following problems will arise. Once the disk and the actuator are assembled within a housing, the shrouded portion will be an obstacle and it will be impossible to move the actuator in a direction indicated by a broken line 80 with respect to the disk. To avoid such inconvenience will require a special assembly method in which the disk and the actuator are dropped together into the housing while maintaining the actuator over the disk. This fabrication method is very intricate and it is anticipated that expensive equipment will be necessary.
An object of the present invention is to provide a shroud near the circumferential edge of a disk after assembly.
Another object of the present invention is to suppress disk flutter and enhance track positioning precision.
Still another object of the present invention is to reduce power dissipation and acoustic noise by preventing turbulent flow caused by an increase in the number of revolutions of a disk.